Abstract

Laterite profiles in the Nilambur gold prospect of southern India contain dust, grains, and nuggets of supergene gold. Whereas chemical rounding and corrosion cavities characterize the gold grains in upper zones of intense weathering, the lower, moderately to slightly weathered horizons contain grains with jagged contours and less common etch pits. Scanning electron microscopy reveals a variety of incipient growth patterns of secondary gold, including spongy, filamental, petalitic, or dendritic patterns. Spectacular examples of "painted" gold with a bright luster that precipitated over dull primary grains are documented. Electron microprobe analyses of secondary gold grains show up to 99.93% Au and exceedingly high fineness values (i.e., 1000 Au/Au+Ag) ranging from 991 to 999, compared to the normal fineness of 950 reported for supergene gold from other terrains. Particle-induced X-ray emission analyses indicate that Fe is the commonly associated trace metal, thus linking supergene gold genesis in Nilambur to a model involving ferrolysis, whereby mobility of gold in weathering profiles was controlled by pyrite oxidation. The nature of occurrence, grain morphology, textural parameters, and unusual fineness of gold grains in Nilambur laterites testify to distinct low-temperature processes attending the chemical dissolution, migration, and reprecipitation of gold during tropical weathering.

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